Items often need to be packaged for shipment. Typically, items are placed in containers or boxes, and the containers are stacked on a pallet. The containers are tied down with straps to prevent the containers from moving during shipping. Round containers in particular are difficult to stack on pallets.
For certain applications, such as shipping explosive items, the containers must maintain a seal against air and liquids and must be preloaded to contain a minimum pressure within the container. Typically, metal containers are used for this purpose, because these requirements are more readily achieved with metals. Composite materials have generally not been used, because the seal between the end of the container and the closure is more prone to failure, such as from brooming or fraying of the fiber reinforcement of the composite material.